This invention relates to drawing or drawing and ironing a workpiece. More particularly, it relates to a method and the apparatus for continuously measuring the axial forces and bending moments imposed on the nose of a punch and on the punch base as the punch forces the workpiece through a draw die or ironing die. Measuring such forces makes it possible to determine 1) the axial force sustained by the just-formed workpiece sidewall emerging from the die, and 2) the friction generated along the interface of the punch and the workpiece sidewall.
When a metal blank is drawn or drawn and ironed into an open end article, such as a can body, it is desirable to maximize efficiency of material usage in the part while minimizing the percentage of defective parts produced.
A common defect resulting from the drawing and ironing processes is fracture of the sidewall at the die exit. A number of variables affect the forming stresses and strains which control fracture. Examples of such variables are the microstructure and properties of the metal being formed, and process variables such as lubrication and the reductions in diameter and sidewall thickness required to form the finished article. For example, if the percent draw reduction (diameter reduction) or ironing reduction (sidewall thickness reduction) attempted in an operation is increased while leaving other variables unchanged, higher forming strains and stresses result. If the material and process are such that the sidewall's tensile stress at the die exit exceeds its ultimate strength, the sidewall will fracture. Reducing the exiting sidewall stress, therefore, lessens the probability of a fracture. Fractures may indicate that a modification in the forming procedure or the workpiece material is desirable to efficiently produce the article. Knowledge of the exiting sidewall's tensile stress under a range of forming process conditions, therefore, is beneficial in the design and selection of workpiece and tooling materials and surfaces, lubrication and other process variables.
In forming can bodies by drawing a cup followed by redrawing and one or more ironing operations, efforts have been made to determine the reformed sidewall's strength and the stress it sustains during deformation of the remainder of the sidewall. This tensile stress in the reformed sidewall varies along its length with its magnitude being the greatest at the die exit, where fractures often occur; sidewall strength provides a measured limit on this sidewall stress. Due to friction between the punch and reformed sidewall, tension decreases along the sidewall from a maximum at the die exit to a minimum at the sidewall juncture with the can bottom.
One method of determining allowable stresses in the sidewall has been to conduct uniaxial tension tests on sidewall specimens. This method is unsatisfactory since uniaxial tension properties of a sidewall specimen from a single location are not indicative of the actual sidewall forming stress or ultimate strength in drawing and drawing and ironing processes. The strain states in these processes are not uniaxial tension; ironing causes through-thickness plane-strain compression, in contrast to the in-plane stretching imposed by uniaxial tension tests. Since the length of a specimen required to perform a uniaxial tension test approaches or exceeds the height of the sidewall (depending on the forming stage in can making), only one uniaxial tension specimen can be taken at any circumferential location. The mechanical properties of the article both before and after reforming may vary around its circumference and along its length, however, due to the material's anisotropy imposed by prior processes in making the sheet and strain distribution from the article's manufacture. A uniaxial tension test of a sidewall specimen may not be indicative, therefore, of the yield stress in forming or the limiting stress which it may sustain. Neither can tension carried by the sidewall during forming be predicted from its incoming uniaxial yield strength, since process variables such as die profile, percent thickness reduction and frictional conditions and their variation around the circumference greatly influence this forming stress. Fracture occurs in uniaxial tension tests at thickness strains much below those routinely achieved in ironing, emphasizing the dissimilarity of the two deformation processes. Also, the fracture in the uniaxial tension test cannot be related to an actual failure location in can forming. For these reasons, the relevancy of yield and ultimate tensile strengths measured from uniaxial tension tests is doubtful. Forming and fracture stresses determined from direct measurements would provide more accurate, applicable information concerning the material, the process and the nature of the failure.
Another method which has been used in an effort to determine stress in the sidewall at the die exit involves measuring the punch base force during forming using a compressive load cell mounted between the press ram and the punch base. Punch base force is the axial force required to push the punch and workpiece through the die. The tensile stress in the exiting sidewall is computed as the measured punch base force divided by the cross-sectional area of the sidewall at the die exit. The value of this force measured at fracture is similarly used to determine the ultimate tensile strength of the sidewall at the fracture location. These calculated stresses significantly overestimate the sidewall stress at the die exit. The true sidewall tension is substantially less than the punch base force due to punch friction, which acts on the entire inner surface of the workpiece in the direction of punch motion. Although the known foregoing methods of determining stress in the sidewall have been beneficial in helping to select suitable materials and processing parameters, it may be seen that these methods do not yield results which accurately reflect the conditions under which an object is stressed during drawing or drawing and ironing. By a method of this invention, the axial forces on the punch base and nose are continuously measured as the punch forces a blank through one or more dies in drawing or drawing and ironing a workpiece. From these forces the actual tensile stress in the sidewall of the workpiece can be determined with a relatively high degree of accuracy.
In addition to measuring the axial forces on the punch nose and base, this invention includes a method to describe the distribution of sidewall tension and friction stresses around the circumference of the article, providing valuable information regarding the nonuniformity of the process conditions, e.g., lubrication, thickness reductions (punch/die alignment), etc., and regarding the nature and circumferential location of a fracture's initiation point.
This invention may be further adapted to measure the amount of bending of the punch near its nose and its base. In any process to form or reform a hollow article with a punch and die, any process or material nonuniformity around the periphery of the punch may result in imbalanced lateral and/or axial loading, applying a bending moment to the punch at various locations. For instance, process or material nonuniformity may produce, around the circumference, a variation in reformed sidewall tension at the punch nose; as a result, the punch nose will be unevenly loaded, generating a bending moment on the punch nose. By detecting this bending at the punch nose, the variation (both magnitude and orientation) in sidewall tension at the punch nose can be determined.
Similarly, process or material nonuniformity may produce variation (around the circumference) in the die friction and/or die pressure; as a result, a bending moment will arise at the punch base. This uneven loading will also cause a lateral deflection of the punch. By detecting the bending at the punch base, the imbalance in die friction and pressure which caused it can be determined. In addition, this invention proposes determining, from bending measured at the punch base, the magnitude of the lateral deflection of the punch as it travels through the die(s).
The variation in punch friction around the periphery of the punch may also be derived from variations in punch nose load and punch base load thus determined. The bending detected at the punch base may be thought of as a variation in punch base load around the circumference of the punch base. The punch base load value is subtracted from the corresponding punch nose load value for the same circumferential location, thereby calculating local values of punch friction.
This invention also proposes monitoring the axial forces and bending the punch nose and base in a production press, e.g., a can body forming press, to provide feedback signals for a computer-based process control system to indicate departure from safe ranges of sidewall tension and force distribution. Further, punch lateral deflection is monitored to provide feedback to indicate actual punch to die misalignment as the punch travels through the die(s) during drawing or drawing and ironing. This can permit real-time corrections of process variables or prompt needed press or tooling maintenance to decrease the total sidewall stress or excessive stress in a specific circumferential location before fractures occur to disrupt production. For example, aluminum pickup or wear on the punch or die causes plowing of longitudinal scratches in the inner or outer surfaces of the finished can, producing undesirable surface finish and cleaning problems. pickup or wear on the tools may cause higher forming forces which may generate higher sidewall stresses and produce stress concentrations in the scratches in the exiting sidewall. Both the generation of high sidewall stresses and stress concentrations are conditions that might lead to sidewall fractures. Progressive die pickup and wear may be detected as an increasing trend over time in total punch load and sidewall tension. Punch pickup or wear may be detected as an increase in punch friction. Sidewall tension and punch friction measurement, therefore, by a method of this invention, can indicate the need to replace worn punches and dies, e.g., draw, redraw and ironing dies.
Apparatus and a method of this invention for measuring the force and bending at the punch nose and at the punch base may be advantageous in a number of ways for any metal forming procedure which requires forcing a workpiece through a die with a punch, producing a hollow article. For example, when forming a deep-drawn article from flat sheet, determining such forces may be helpful in selection or evaluation of workpiece material or process variables such as the percent reduction in diameter to be made in a single draw. As another example, determining forces on the punch at sidewall fracture is helpful in determining the optimum amount of ironing that can be tolerated by a given material. Since the sidewall thickness is severely reduced during ironing, large tensile forces are generated in the reformed sidewall which can exceed its ultimate strength, resulting in a fracture. The ultimate tensile strength of the sidewall material in undergoing this process can be determined from measurement of the sidewall force at fracture by the method of this invention. To determine the ultimate strength, a fracture may be induced through modification of one or more process variables, e.g., greater thickness reduction. One efficient means to induce a fracture by ironing to excessive thickness reductions employs a punch which tapers slightly along part or all of its cylindrical surface with its greatest diameter near the base. Ironing with this tapered punch imposes ever-increasing thickness reductions and stresses along the sidewall of each workpiece until fracture occurs. A method of employing a tapered punch to induce fracture, and thus, investigate maximum ironing reductions, is the subject of a copending patent application Ser. No. 839,782, filed Mar. 14, 1986. Measuring sidewall tension by the method of this invention to determine fracture stress adds to the value of testing with a tapered punch. In this way, an upper limit is established on tolerable sidewall tension for the workpiece material tested. Similar tests can then be run with a punch of uniform outer diameter at different levels of each process variable, which may or may not regularly cause a fracture, to determine the resulting measured sidewall tension. Thus, the process can be utilized for establishing the optimum conditions for a variety of process variables. For example, the greatest practical thickness reduction can be determined by increasing reduction using a series of dies of smaller inner diameters until the resulting tension reaches a chosen percentage (less than 100) of the sidewall strength previously determined. Alternately, for the required reduction, the levels of process variables, such as die entry angle, can be optimized to minimize measured sidewall tension (and fracture frequency).